1. Field of the Invention
This invention relates to (meth)acrylate-based polymerizable compositions and adhesive systems prepared therefrom, which include an alkylated borohydride or tetraalkyl borane metal or ammonium salt and a polymerizable siloxane. The inventive compositions and adhesive systems are particularly well suited for bonding applications, which involve at least one low energy bonding surface, for example, polyolefin surfaces, such as polyethylene and polypropylene.
2. Brief Description of Related Technology
Low energy surfaces such as polyolefins, i.e, polyethylene, polypropylene, polybutene, polyisoprene, and copolymers thereof, are well known to be difficult to bond to each other and to other surfaces using adhesive bonding technology since they have few active bonding sites available at the free surfaces. Low energy surfaces typically have surface energy values of less than 45 mJ/m2, more typically less than 40 mJ/m2, such as less than 35 mJ/m2.
Bonding low energy surfaces by surface pre-treatments such as flame treatments, plasma treatments, oxidation, sputter etching, corona discharge, or primer treatments with a high surface energy material is well known. Such treatments disrupt the bonds at the surface of the low energy material providing sites which are reactive and which can participate in bonding reactions with adhesive materials. However, such surface pre-treatments are usually undesirable, in that they add cost to the process, they are not particularly reproducible in their results, and the effect of the pre-treatments wears off with time so the pre-treated surfaces must be re-pre-treated if they are not bonded within a reasonable period of time.
The chemistry of organic boron compounds has been studied in detail [see e.g. D. Barton and W. D. Olliis, “Comprehensive Organic Chemistry”, Vol. 3, Part 14, Pergamon Press (1979) and H. C. Brown, Boranes in Organic Chemistry, Cornell University Press, Ithaca, N.Y. (1972)]. The use of organoboranes such as the trialkylboranes including triethylborane and tributylborane for initiating and catalyzing the polymerization of vinyl monomers is well known. However, such organoborane compounds are known to be flammable in air so that the compounds and compositions containing them require special handling and the compositions have poor shelf stability [see e.g. U.S. Pat. No. 3,236,823 (Jennes), and the Background section of U.S. Pat. No. 5,935,711 (Pocius), at col. 2].
Certain boron alkyl compounds and their use as initiators of polymerization are described in U.S. Pat. Nos. 4,515,724, 4,638,092, 4,638,498, 4,676,858 and 4,921,921, each to Ritter.
U.S. Pat. Nos. 5,106,928, 5,143,884, 5,286,821, 5,310,835 and 5,376,746, each disclose a two-part initiator system for acrylic adhesive compositions, in which the first part includes a reportedly stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde.
Japanese Patent Publication No. S48-18928 describes a method for adhering polyolefin or vinyl polymers using an adhesive obtained by adding trialkylboron to a vinyl monomer, with or without vinyl polymer. Examples of trialkylboron include triisopropylboron, tri-n-butylboron, tripropylboron and tri-tert-butylboron.
U.S. Pat. No. 3,275,611 (Mottus) describes a process for polymerizing unsaturated monomers with a catalyst comprising an organoboron compound, a peroxygen compound and an amine complexing agent for the boron compound.
It is well known that the bonding of polyolefin substrates and other low surface energy substrates causes particular difficulties. Attempts have been made to overcome these difficulties by the extensive and expensive substrate surface preparation described above, or by priming the surface with a high surface energy primer. However, it is desired to develop adhesive compositions, which will bond low surface energy substrates without such surface preparation.
U.S. Pat. No. 5,539,070 (Zharov), and U.S. Pat. Nos. 5,616,796, 5,621,143, 5,681,910, 5,684,102, 5,686,544, 5,718,977, 5,795,657 and the '711 patent describe organoborane amine complexes which can be used in systems that initiate the polymerization of acrylic monomers in compositions useful for bonding low surface energy plastics substrates such as polyethylene, polypropylene and polytetrafluoroethylene.
International Patent Publication No. WO 99/64528 describes low odor polymerizable compositions comprising monomer blends and organoborane amine complex initiators. These systems require the preparation of trialkyl borane amine complexes to achieve the desired performance and shelf stability. The manufacture of such complexes is an undesirably complicated process. Further, the presence of the amines results in cured adhesives that have a tendency to become yellow in color on aging.
International Patent Publication No. WO 01/44311 also describes amine organoborane complex polymerization initiators in bonding compositions for low surface energy substrates.
International Patent Publication No. WO 01/32716 (“PCT '716”) acknowledges that while complexes of an organoborane and an amine may be useful in many applications, certain problems may arise due to the use of amine complexing agents in such conventional complexes. For example, when the complexes contain a primary amine, adhesives prepared therefrom may be prone to discoloration, such as yellowing; further, when including reactive diluents, such as aziridines described, for example, in International Patent Publication No. WO 98/17694, in compositions containing the complexes, the diluents may prematurely react with protic amines (i.e., those amines in which a nitrogen atom is bonded to at least one hydrogen atom) in such complexes, prematurely decomplexing the organoborane initiator.
PCT '716 therefore proposes a complex of an organoborane and a complexing agent of at least one hydroxide or alkoxide, particularly a complex represented by the formula

where R1 is an alkyl group having 1 to about 10 carbon atoms; R2 and R3 may be the same or different and are selected from alkyl groups having 1 to about 10 carbon atoms and phenyl-containing groups; the value of “v” is selected so as to provide an effective ratio of oxygen atoms of the alkoxides and/or hydroxides to boron atoms in the complex; each R4 is independently selected from hydrogen or an organic group (e.g., an alkyl or alkylene group); M(m+) represents a countercation [comprising a monovalent cation, such as a Group IA metal (e.g., lithium, sodium and potassium) cation or onium, or a multivalent cation, such as a Group IIA metal (e.g., calcium and magnesium) cation]; n is an integer greater than zero; and m is an integer greater than zero. Particular complexing agents are stated to have a countercation selected from sodium, potassium and tetraalkylammoniums. When any R4 is hydrogen, the complexing agent is said to comprise at least one hydroxide. When any R4 is an organic group, the complexing agent is said to comprise at least one alkoxide.
According to PCT '716, the complexing agent (i.e., the hydroxide or alkoxide) is used in the form of a salt. That is, the complexing agent is stabilized by a suitable countercation such that the complexing agent is capable of complexing the initiator. Thus, in the Formula, M(m+) represents a countercation that stabilizes the complexing agent, not a cation that forms an ionic compound with the organoborane initiator.
PCT '716 states that hydroxides and alkoxides provide strong coupling to organometallic initiators, such as organoboranes, with the resulting complexes having excellent oxidative stability. Thus, complexing agents of at least one hydroxide, alkoxide, or mixtures thereof are said to be particularly beneficial. Such a complex is stated to be a tightly co-coordinated salt formed by association of a Lewis acid (the initiator) and a Lewis base (the hydroxide or alkoxide). This indicates that the oxygen atom of the alkoxide or hydroxide is bonded or co-coordinated to the boron atom of the initiator.
Tetraorganylborate salts of tetraalkylammonium, sodium or lithium are known as photoinitiators in photocurable compositions for imaging materials (see e.g. U.S. Pat. Nos. 4,950,581, 6,110,987 and 6,171,700). Tetraorganylborate salts do not however have a boron-hydrogen bond. The distinction between boron-hydrogen compounds, triorganylboranes and organoborate salts is well illustrated in D. Barton and W. D. Ollis, “Comprehensive Organic Chemistry”, to which Chapters 14.2, 14.3 and 14.4 of Vol. 3 are devoted.
More recently, Loctite (R&D) Ltd. has designed and developed technology that is described in International Patent Publication Nos. WO 02/34851, WO 02/34582 and WO 2003/089536 relating generally to polymerizable adhesive compositions with a free radically polymerizable component and an initiator system of an alkyl borohydride with either a metal or ammonium cation. These compositions also describe the use of aziridine components.
And Henkel Corporation together with Loctite (R&D) Ltd. have designed and developed technology, which include a carrier to such polymerizable adhesive compositions so that the compositions have a flash point above 140° F. See International Patent Publication No. WO 03/035703.
The use of aziridines generally however are under regulatory scrutiny, and thus labeling requirements are imposed, which may be an impediment to wide spread usage.
Despite the work of many in this general field of bonding low energy services, there is a need for polymerizable compositions for bonding low surface energy substrates, such as polyolefins, and for end users to have a variety of such compositions which achieve that result through different technical strategies.